This invention relates to magnetic bubble memories; and more particularly it relates to mechanisms for detecting the presence of magnetic bubbles in such memories.
Some of the standard components of a bubble memory are a planar magnetic film in which the bubbles reside, an insulating layer over the film, a serial string of asymmetric chevrons on the insulating layer which define propagate paths along which the bubbles move, and a magnetic field which rotates in the plane of the film to cause the bubbles to move along the propagate paths. Also, a mechanism is disposed on the insulating layer for detecting the bubbles as they move along a portion of the bubble propagate path.
Reference numeral 20 of FIG. 1 indicates a prior art mechanism for detecting bubbles. Mechanism 20 has one part which is called a detector 21, and it has another part which is called a dummy detector 22. Detector 21 and dummy detector 22 both lie on an insulating layer 23 which in turn lies on a film which contains the bubbles.
Detector 21 includes approximately thirty rows of unconnected symmetric chevrons 21a. They are illustrated in the blown-up portion of FIG. 1; and they operate to stretch bubbles that are received from a bubble propagation path 24. Between two hundred and three hundred chevrons are included in each row.
Lying next to the chevron rows 21a is a single row of interconnected symmetric chevrons 21b. They are made of a magnet. Lying next to row 21b are approximately four rows of unconnected symmetric chevrons 21c. They operate to move the stretched bubbles away from row 21b and to annihilate the bubbles along the last row (i.e., the row that is furthest from row 21b).
Dummy detector 22 is separated from detector 21 by a space which approximately three rows of chevrons would occupy. This space is provided so that detector 21 and dummy detector 22 do not electromagnetically interact.
Dummy detector 22 includes approximately 10 rows of unconnected symmetric chevrons 22a. These chevrons 22a do not receive any bubbles from a bubble propagate path such as path 24.
Lying next to the chevron rows 22a is a single row of interconnected symmetric chevrons 22b. This row is made of a magnetoresistive material; and since the chevrons 22a never receive any bubbles, the resistance of the chevrons 22b operates as a reference which indicates the absence of a bubble. Conductive leads 22' connect to row 22b so that its resistance can be detected external to the chip.
Lying next to row 22b are approximately four rows of chevrons 22c. Their function, along with chevrons 22a, is to make the physical structure of dummy detector 22b similar to the structure of detector 21 so that, ideally, the resistance of the rows 21b and 22b will be the same when no bubble is being sensed by detector 21.
One problem, however, with the above-described prior art mechanism 20 is that it occupies too much space. Typically, the area occupied by the dummy detector 22 is at 3 least width of chip X 10 mils. That area could be used to store bubbles and thereby increase the memory's storage capacity if it were not needed for the dummy detector.
Another problem with mechanism 20 is that the resistance of the interconnected chevron rows 21b and 22b is sensitive to the pattern of the bubbles being detected. In other words, the resistance in chevron row 21b is not determined solely by the presence or absence of a bubble under that row; but instead, it is also determined by the presence or absence of a bubble in the rows which lie alongside of row 21b. Thus, if no bubble is beneath row 21b while bubbles are beneath all of the other rows 21a and 21c, the resistance of row 21b will be substantially different than the resistance of row 22b.
Still another problem with the mechanism 20 is that the resistance of chevron rows 21b and 22b is affected differently by the magnetic field which rotates in the plane of the film. This is because rows 21b and 22b lie apart from one another, and because the strength of a rotating field is stronger at the center of a chip than at its periphery. And since the chevron rows 21b and 22b are made of a magnetoresistive material, the resistance of those elements is determined not only by the presence or absence of a bubble; but it is also determined by the strength of the rotating magnetic field.